A typical optical storage system uses an optical head to focus a monochromatic optical beam to a small spot on a recording layer for reading and writing. The optical head may be positioned over the medium by a spacing greater than one wavelength, i.e., in a "far-field" optical configuration where the optical energy is coupled between the optical head and the medium by light propagation. An optical head with a large numerical aperture can produce a small spot size which has a lower limit on the order of one half wavelength due to the diffraction limit. The areal density of an optical storage device, hence, is limited by this diffraction-limited spot size.
An optical storage system can also operate in a "near-field" configuration where the optical head is spaced from the optical medium by a distance on the order of or less than one wavelength. The optical coupling between the optical head and the medium, therefore, can be effected by evanescent coupling, with or without light propagation. An effective numerical aperture of the optical head in such a near-field configuration can be greater than unity. Hence, a near-field optical storage system can achieve a focused beam spot size much less than one half wavelength and to realize a high areal storage density.
The optical head is a critical part of an optical storage system and its properties can significantly affect the overall performance of the system. In addition to the focusing of a read/write beam, the optical head also controls other operations of the system, including recording of data, signal detection, beam tracking on the data tracks, and grey code detection.
For example, many optical heads have an optical interfacing surface that couples optical radiation to and from the storage medium. This interfacing surface may be an optical surface of a lens, a small optical flat, or a transparent mesa formed as part of a lens. One technical challenge is to maintain this interfacing surface free of contaminants which may be formed on the interfacing surface from a variety of sources. One source is localized heating at and near the focused spot on the recording medium surface by absorption of a focused optical beam. Certain species on the recording medium surface may become desorbed due to the localized heating and transfer to adhere to the optical head. Other sources include material deposited on the optical head through intermittent contacts between the optical head and the medium surface, and particulates present in the disk drive.
Contaminants adhered to the interfacing surface in the path of the optical signals can adversely affect the signals by causing signal distortions. Such signal distortions, in turn, can lead to loss of tracking, track misregistration, data jitter, reduction in the signal-to-noise ratio, or other problems that degrade the performance or even cause malfunction of the system.